背景图片一般最后加载，先加载图片可能会导致主页打开慢的情况。 先加载主体容器后加载图片分两种情况： 1.主体容器加载背景图片，那么后背景图片加载主体会闪一下。这个用动画也是过渡不了的。 2.主体容器与背景图为兄弟关系，那么主体要用绝对定位等方式覆盖在背景图上层

dom 结构

<div>
    <div class="bg-image"></div>
    <div class="main-container">
        /** main content**/
        <div></div>
    </div>
</div>

css样式

//主体容器背景用css
.main-container{
    background-color:rgba(255,255,255,0)
}
//背景图容器css
#bg-image{
    width: 100%;
    height: 100%;
    overflow: hidden;
    position:absolute;
    top:0;
    left:0;
    background-size: cover;
    background-position: 50% 50%;
    background-repeat: no-repeat;

    //opacity animation
    opacity:0;
    animation-delay: 0s;
    animation-duration: 1s;
    animation-fill-mode: forwards;
    animation-timing-function: ease-in;
}
//关键帧动画
@keyframes load {
    0% { opacity: 0; }
    100% { opacity: 1; }
}

延时加载图片

    //create image
    const domImage = new Image()
    domImage.src = "https://cn.bing.com/ImageResolution.aspx?w=375&h=812&hash=4c53f273ecb43b0f5ed8318a5684a324"
    //create image load event
    domImage.onLoad = ()=>{
        let bg = document.querySelector("#bg-image")
        bg.style = `
          background-image:url(${background.src});
          animation-name: load;
        `
    }